SAS program to generate confidence intervals for treatments in
a cross-over with two experimental treatments and a control
treatment.

Use your own data step with your own data, then copy the proc
mixed part of this program to analyze them. Delete "by sim".

Main finding: for reliability=0.95, a sample size of 12 gives
a confidence interval of about 0.38 effect-size units, which is an
acceptable width for the smallest effects (see
Sample Size On The Fly for further
discussion about acceptable confidence intervals). The model using
the trial effect has a slight advantage over the model with the group
effect for a compound-symmetric covariance matrix, but the advantage
is greater with an unstructured covariance matrix (CI of 0.40 vs
0.44).

WARNING: To those using trial and treat as
within-subject measures (the recommended method)... If you use a
covariance structure other than compound symmetry, you should specify
whether you want trial or treat as the repeated measure that SAS uses
to model the covariance structure. Do it in the repeated statement.
In general choose treat, because if there is a problem with different
variances (lack of sphericity), it will be because of individual
differences in the response to the treatments. But it is possible
that subjects are more variable in their response to different trials
(first, second, third...) rather than different treatments, if, for
example, the outcome measure is the response to a difficult test that
some subjects master quicker than others. I guess you should try both
and examine the covariances. You might need sophisticated help
here.

If you don't specify a repeated effect, SAS seems to decide
which one to use on the basis of the sort order and completeness of
the data set, not on the basis of the order of the terms in the class
or model statements.